1. Field of the Invention
The present invention relates to a toner, a method for producing the toner and a developer.
2. Description of the Related Art
Conventionally, in electrophotographic apparatuses and electrostatic recording apparatuses, electric latent images or magnetic latent images are made to visible images by using a toner. For example, in electrophotography, a latent electrostatic image (latent image) is formed on a photoconductor, and the latent image is developed using a toner so as to form a toner image (visible image). The toner image is generally transferred to a recording medium such as a paper, and then fixed, for example, by heating. The toner particles used for a latent electrostatic development are generally colored particles in which a colorant, charge controlling agent, and other additives are contained in a binder resin.
For a fixing method based on the dry development, an image fixed by using a heating roller is generally used for favorable energy efficiency. Moreover, in recent years, for saving energy by fixing a toner at low-temperature, there is a tendency that the heat energy required to be given to the toner at the time of fixing is low. In DSM (demand-side management) programs of the International Energy Agency (IEA) in 1999, there is a project for procuring technologies of the next generation copiers, and requirement specifications thereof have been disclosed. For a copier of 30 cpm (copies per minute) or more, save of a significant amount of energy as compared to the conventional copiers is required to be accomplished such that the stand-by time is 10 seconds or less, and power consumption during the stand-by time is 10 watts to 30 watts (varies depending on a copying speed). One of the methods for achieving the requirement is a method of improving a temperature response of toner by lowering heat capacity of a fixing member such as the heating roller etc. However, this method does not sufficiently satisfy the requirement.
To satisfy the requirement and minimize the stand-by time, it is considered that an essential technical requirement is that the melt starting temperature of toner is lowered so as to lower the toner-fixing temperature when the machine is in use. In order to deal with such low-temperature fixing, attempts have been made to use a polyester resin having an excellent low-temperature fixing property and comparatively favorable heat resistance and storage stability, instead of a styrene-acrylic resin which has been conventionally used.
As for the fixing system in the electrophotography, a heat roller fixing system is widely used for its high energy efficiency and in view of device miniaturization, in which system a heating roller which is excellent in heat efficiency is directly pressed against a toner image on a recording medium for fixing. Considering the environment-friendly policy including energy-saving, lower power consumption is desired for the heating roller in the fixing step.
In attempts to solve the above problem, fixing units have been improved and rollers have a reduced thickness on the side in contact with the toner image carrying surface for further increasing heat energy efficiency, realizing a significant reduction in start-up time. However, the reduced specific heat capacity has caused a difference in temperature between the area where the recording medium passes through and the area where the recording medium does not. Then, a so-called hot offset phenomenon occurs in which toner melts and adheres to a fixing roller and, after one rotation of the fixing roller, this toner is fixed to non-image areas on the recording medium. Therefore, there is a severe demand for toner on hot offset resistance as well as low-temperature fixing property.
Methods for producing a toner for developing a latent electrostatic image are broadly classified into pulverization methods and polymerization methods. In the pulverization method, a colorant, charge controlling agent, anti-offset agent, and the like are melted, mixed and uniformly dispersed in a thermoplastic resin to obtain a toner composition, and then the composition is pulverized and classified to thereby produce a toner. According to the pulverization method, it is possible to produce a toner having excellent properties to some extent, however, there are limitations on selection of toner materials. For example, a toner composition produced by melting and mixing toner materials are required to be pulverized and classified by using an economically available apparatus. To respond to the request, the melted and mixed toner component is forced to be made sufficiently brittle. For this reason, when the toner composition is pulverized into particles, a particle size distribution is liable to be broad. When a copied image having excellent resolution and gradation is expected to be obtained, for example, it suffers from the disadvantages that fine particles each having a particle diameter of 4 μm or less and particles each having a particle diameter of 15 μm or more must be eliminated by classifying the toner particles, thereby causing substantially low toner yield. In addition, in the pulverization method, it is hard to uniformly disperse a colorant and charge controlling agent, and the like in a thermoplastic resin. A dispersion solution in which components are insufficiently dispersed adversely affects flowability, developing property and durability of a toner, image quality, and the like.
Recently, in order to overcome the problems in the pulverization method, a toner production method by means of a polymerization method has been proposed. For example, a toner is produced by a suspension polymerization method, emulsion polymerization aggregation method and the like. However, it is difficult to produce a toner using a polyester resin which is excellent in low-temperature fixing property.
To solve these problems, it is known a dry toner consisting of particles formed by the elongation reaction and/or crosslinking reaction of an isocyanate group-containing prepolymer (A) with amines (B) in an aqueous medium (JP-A No. 11-149180). Such a toner is relatively excellent in low-temperature fixing property by using a polyester resin as a binder resin. However, the requirement to hot offset resistance is not satisfied only by containing high molecular mass component in the binder resin and it is necessary to contain a releasing agent in a toner.
In attempting to improve hot offset resistance, the releasing agent preferably has low melt viscosity and excellent separation property from a resin. Examples of generally known releasing agents used in a toner are, for example, carnauba wax, montan wax (Japanese Patent Application Laid-Open (JP-A) Nos. 1-185660, 1-185661, 1-185662 and 1-185663), hydrocarbon wax such as polyethylenes, polypropylenes and paraffins (Japanese Patent Application Publication (JP-B) Nos. 52-3304 and 52-3305). Of these, hydrocarbon wax is significantly effective to improve hot offset resistance because it has polarity largely different from a polyester resin, excellent separation property between an image and a fixing member and low melt viscosity, and quickly exudes from a toner to the fixing member.
On the other hand, in a toner obtained by emulsifying or dispersing a solution containing a material constituting the toner (toner material solution) in an aqueous medium, it has revealed that the most frequent volume particle diameter of a dispersoid of the toner material solution influences uniformity in compositions and particle diameters of base particles to be obtained (see JP-A 2006-293309). At the same time, the releasing agent contained in the toner material solution significantly influences uniformity of base particles. Uneven compositions and particle diameters of the base particles cause wide variation of charging ability, flowability and fixing property of the toner, and easily cause fog image by uncharged toner, toner scattering to a non-image part, occurrence of background smear, adhesion of a toner constituent to a developing part, and offset to a fixing member. Thus, a stable and high quality image is hard to be obtained. Therefore, the releasing agent is desired to be finely dispersed.
However, it is very difficult to finely disperse the hydrocarbon wax in the toner material solution, because the polarity of the hydrocarbon wax is largely different from those of an organic solvent and a polyester resin, and a functional group having compatibility is not present between the hydrocarbon wax and the polyester resin. Moreover, a great amount of energy is needed to improve dispersibility, because a dispersion step takes an extremely long time. Additionally, production of a toner having a constant quality for a long period is difficult because the releasing agent easily aggregates over time in the toner material solution.
Moreover, an organic solvent having relatively high polarity and low boiling point is frequently used, because it has high solubility to polyester resins and can be easily removed. Of these, as a main component, ester solvents such as ethyl acetate and ketone solvents such as methyl ethyl ketone are frequently used. However, when such an organic solvent is used, compatibility between the organic solvent and the hydrocarbon wax tends to be further decreased, thus it becomes more difficult to finely disperse the hydrocarbon wax in the toner material solution.